The electrochromism of inorganic, organic small molecule, and polymeric based materials has demonstrated the possibility of wide and lucrative applications ranging from displays, smart windows, E-paper, and E-cloth, to dual use emissive/nonemissive systems. Of the materials listed, conjugated polymers hold significant promise as recent advances have demonstrated that they have straightforward synthesis and color tuning, coupled with potentially low cost processability. The solubility of electrochromic polymers (ECPs) allows for high throughput methods such as spray casting and slot-die coating for large area roll-to-roll processing from aqueous or organic solutions for both display and window applications, necessary for large scale printing and patterning of EC devices.
Cathodically coloring polymers have been formed that possess vibrantly colored neutral and transmissive oxidized states. Polymers that switch from a colored state to a fully transmissive state are required for applications in window and display technologies where multicolor polymers are not appropriate or desired. The electrochromic polymer, ECP-Yellow, herein referred to as ProDOT-Ph, as well as other alternating copolymers disclosed in Amb et al. U.S. Pat. No. 8,399,603, allowed the completion of a full color palette of solution processable ECPs and demonstrated the importance of obtaining a vibrant yellow-to-transmissive electrochrome as this completes both subtractive primary color sets: cyan-magenta-yellow (CMY) and red-yellow-blue (RYB). Through the use of subtractive color mixing, a wide variety of colors can be obtained for use in full color displays. These yellow ECPs are unique in that they are solution-processable cathodically-coloring, yellow-to-transmissive ECPs that switch at oxidation potentials that are relatively high compared to many other electron-rich ECPs.
These relatively high potentials complicate use with multiple ECPs on the same electrode as over-oxidation of more easily oxidized polymers can occur when applying the potential to switch ProDOT-Ph to a fully transparent state. Hence, the identification of conjugated polymers with lower oxidation potentials to achieve high transparency in the bleached state, while maintaining a vibrant yellow neutral state is desirable.